Interface effects and the work of fracture of a fibrous composite

Abstract

The stresses near the tip of a crack which lies normal to a set of alined fibres is discussed when the elastic properties of the composite are appropriate to those of a carbon reinforced epoxy resin. Splitting parallel to the fibres is expected to occur before fibre fracture only if an interface parallel to the fibres of one fiftieth the strength of the composite parallel to the fibres is present. The two mechanisms of energy dissipation which have been suggested to occur in a fibre composite of brittle fibres in a brittle matrix, namely pull out and debonding, are discussed. Experiments to measure the latter are described. The work of debonding (< 10$^{4}$ J m$^{-1}$) is usually less than the work of pull out. The theory of pull out is described and experiments to support it noted. An important result is that the work of pull out increases linearly with fibre diameter, and is likely to be inconveniently small for fibres of diameter $\lesssim $ 10 $\mu $m. The relative advantages and disadvantages of fibres of various diameters are discussed. Fibres of diameter $\gtrsim $ 25 $\mu $m lead to large works of pull out and are stable and easily handled. Thinner fibres (< 10 $\mu $m diameter) give rise to constraint effects which are important in metallic matrices.